1. Field of the Invention
This invention relates to a surface light source device of side light type and more particularly to a surface light source device of side light type wherein uniformity in color characteristic of irradiation light is improved. This invention is applicable particularly to back lighting of a liquid crystal display.
2. Description of the Related Art
The surface light source device of side light type has been employed in, for example, a liquid crystal display apparatus. This device irradiates the liquid crystal display panel from its back surface. This disposition is suitable for thinning the entire shape of the apparatus.
In the surface light source device of side light type, ordinarily, a rod-like light source such as cold cathode tube is employed as a primary light source and disposed on a side of the light guide plate. Irradiation light emitted from the primary light source passes a side end face of the light guide plate so that it is introduced into the inside of the light guide plate. The introduced irradiation light is transmitted inside of the light guide plate. In that process, light emission occurs from a major surface of the light guide plate toward a liquid crystal display panel.
As the light guide plate employed in such a surface light source device of side light type, a type having substantially equal plate thickness and another type in which the plate thickness decreases as it goes far from the primary light source have been well known. Generally, the latter emits irradiation light more effectively than the former.
FIG. 12 is a disassembly perspective view showing the surface light source device of side light type using the light guide plate of the latter. FIG. 13 shows a section along the line Axe2x80x94A of FIG. 12. Referring to FIGS. 12, 13, the surface light source device of side light type 1 comprise a light guide plate 2, a primary light source 3, a reflection sheet 4, a light scattering sheet 5, and prism sheets 6, 7 acting as a light control member. The reflection sheet 4, light guide plate 2, light scattering sheet 5 and prism sheets 6, 7 are laminatedly arranged and fixed by a frame member (not shown).
The light guide plate 2 has two major surfaces. One major surface provides an emission surface 2C and another major surface provides a back surface 2B. An end surface of the light guide plate 2 provides an incidence surface 2A. The primary light source 3 is composed of a fluorescent lamp 8 with a reflector 9 on the back. As well known, the emission light of the fluorescent lamp 8 contains both short-wavelength component (blue component) and long-wavelength component (complementary color component to blue) of visible ray region.
To intensify brightness level and uniformity of the surface light source device, the back surface 2B of the light guide plate 2 provides"" a light scattering surface 2D. The light scattering surface 2D contains a distribution pattern composed of, for example, a great number of light scattering dot-like elements. This distribution pattern is called dot pattern.
FIG. 14 is a view for explaining a basic type of the distribution pattern on the light scattering surface 2D, which depicts the light guide plate 2 viewed from the back surface 2B. In FIG. 14, the light scattering elements are expressed by a great number of large and small squares. The light scattering elements are formed in the form of light scattering ink layer, fine uneven surface or the like. The light scattering ink contains pigment of, for example, magnesium carbonate, titanium oxide or the like. The light scattering property of the light scattering surface 2D tends to increase corresponding to distance from the incidence surface 2A along the lamp 8 to obtain an equal brightness level. This tendency is realized by design of the distribution of size, density or other factors of the light scattering element.
The primary light source 3 is provided with a reflector 9 having a semi-circular section, disposed on the back of a cold cathode tube (fluorescent lamp) 8. The irradiation light is supplied to an end face of the light guide plate 2 through an opening of the reflector 9. For the reflection sheet 4, a sheet-like regular reflection member composed of metallic foil or the like or a sheet-like irregular reflection member composed of white PET film or the like is employed. The reflection sheet 4 brings back light leaking from the back surface 2B to the light guide plate 2 so as to reduce loss of light.
The irradiation light L from the primary light source 3 is introduced into the light guide plate 2 through the incidence surface 2A. The irradiation light L is transmitted toward an end (wedge distal end) while being repeatedly reflected between the back surface 2B along which the reflection sheet 4 is laid and the emission surface 2C. In this while, the irradiation light L is subjected to light scattering action by the light scattering surface 2D formed on the back surface 2B. If the reflection sheet 4 composed of the irregular reflection member is employed, the irradiation light is also subjected to the irregular reflection action.
Each time when the irradiation light L is reflected by the slope 2B, the incidence angle relative to the emission surface 2C gradually decreases. The decrease of the incidence angle increases component having a smaller angle than the optimum angle relative to the emission surface 2C thereby inducing emission from the emission surface 2C. Because this operation is intensified as it goes far toward the end of the light guide plate 2, a shortage of the emission light in a region far from the incidence surface 2A is compensated.
The irradiation light emitted from the emission surface 2C passes the light scattering sheet 5 and is directed toward the prism sheets 6, 7. The light scattering sheet 5 scatters the irradiation light emitted from the light guide plate 2, thereby preventing the light scattering surface of the back surface 2B from being visually noticed from above the emission surface 2C. The light scattering sheet 5 further suppresses local excessive brightness, shade and the like in the light guide plate 2. Depending on the case, the light scattering sheet 5 is not employed.
The prism sheets 6, 7 are formed of transparent sheet-like material of polycarbonate or the like. The prism surface is formed on an opposite side to a surface (outside surface) opposing the light guide plate 2. The prism surface has a great number of projections which extend substantially parallel in a single direction and have a triangle section. In the inside prism sheet 6, its projections are oriented so as to run parallel to the incidence surface 2A. The outside prism sheet 7 is oriented so that the projections extend substantially perpendicular to the incidence surface 2A.
The slopes of these projections correct main emission direction of the emission light to the frontal direction of the emission surface 2C. A double-sided prism sheet having each prism surface on both sides may be used.
Generally, as compared with the surface light source device of side light type in which a substantially equal thickness light guide plate is employed, the surface light source device of side light type employing such a wedge-shaped light guide plate and a prism sheet is capable of emitting the emission light more effectively to the frontal direction.
However, the aforementioned conventional surface light source device of side light type 1 has a problem. That is, there is produced a phenomenon that the color of the emission light changes delicately depending on distance from the incidence surface 2A. This tendency is more remarkable as size of the light guide plate 2 increases.
According to an experiment, as described in FIG. 13, the irradiation light becomes bluish near the incidence surface 2A (blue light component tends to be excessive), and yellowish near the wedge distal end (end of the light guide plate) (blue light component tends to be short). In the middle, the bluish component decreases as it approaches the wedge distal end while the yellowish component increases. In other words, the short-wavelength component relatively decreases as it approaches the wedge distal end from the incidence surface 2A while the long-wavelength component relatively increases.
This phenomenon is estimated to be caused by wavelength dependency of response characteristic (wavelength dependency of reflection or absorption) to light, of elements composing the surface light source device 1. For example, the reflection sheet 4 made of white PET film has reflection factor which varies depending on wavelength, as follows:
wavelength 450 nm; reflection factor 97%
wavelength 600 nm; reflection factor 95.5%
wavelength 750 nm; reflection factor 94%
Because, the material of the light guide plate 2 has generally scattering property (wavelength dependency of refractivity), the optimum angle relative to the emission surface 2C and back surface 2B differs depending on the wavelength. As a result, escaping rate from the light guide plate 2 is provided with wavelength dependency.
It is estimated that as a result of combined actions of these factors, a distribution of the color of the emission light from the light guide plate 2 occurs.
Generally, such a color distribution is not favorable. Particularly, when applied to back lighting of the liquid crystal display, its display quality is remarkably deteriorated.
To solve the above problem, it is therefore an object of the present invention to provide a surface light source device of side light type wherein change in color along distance from near an incidence surface of a light guide plate to an end thereof is suppressed.
The present invention is applied to a surface light source device of side light type comprising a light guide plate having two major surfaces providing an emission surface and a back surface and a primary light source for supplying irradiation light containing short-wavelength component (blue component) and long-wavelength component (complementary color component to blue) of visible ray region through an end face of said light guide plate.
According to the present invention, there is provided a surface light source device of side light type having the above feature, further comprising a color correcting film for suppressing change in color of the irradiation light depending on distance from the end face, disposed in the vicinity of at least one of two major surfaces of the light guide plate.
The color correcting film may be ink printed on the light guide plate. In a case in which a sheet-like member is disposed along the emission surface of the light guide plate, the color correcting film may be ink printed on the sheet-like member. Further if a sheet-like member is disposed along the back surface of the light guide plate, the color correcting film may be ink printed on the sheet-like member. In the ink forming the color correcting film, response characteristics to the short-wavelength component and long-wavelength component contained in the irradiation light are different from each other.
The color correcting film is preferably composed of a great number of film elements. If the ink is applied, a great number of the ink elements are printed on the light guide plate or other sheet-like member.
Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.